In the field of metal packaging for food/beverages, it is well known to seal the access opening of a container body using a foil lid. By “foil” is meant a flexible lidding material including a base layer of metal. The metal base layer provides strength to the lid, forms a barrier to mitigate loss of moisture and flavours from a filled container, and prevents contamination. The foil lid may be bonded to an intermediate metal ring, the ring then seamed to a container body. Alternatively, the foil lid may be bonded directly to the container body as described in WO 2006/092364 A (CROWN PACKAGING TECHNOLOGY, INC) Aug. 9, 2006. As explained below, it is commonplace for dissimilar metals to be used for the foil lid and the ring/container body.
Aluminium is particularly favoured as a material for the base layer of the foil lid because it has a high strength to weight ratio (relative to, say, steel) and can easily be coated with other materials to provide additional properties; for example, with heat seal lacquers to provide heat sealability. Steel is particularly favoured for the ring/container body due to its high strength and relatively low cost. The steel is typically supplied to can makers either as tin-plate (which is steel with a very thin layer of tin electro-deposited onto both sides), or as tin-free steel. For many food and beverage cans it is necessary to coat the metal of the ring/container body with one or more polymer coatings to prevent chemical interactions (e.g. corrosion) occurring between the metal of the ring/container body and the product or external environment. Examples of such polymer coatings include epoxy-based lacquers and polypropylene-based lacquers. In the field of food/beverage packaging, it is essential to reduce/eliminate any corrosion on grounds of hygiene and aesthetics.
The coatings provided on the dissimilar metals of the foil lid and the ring/container body are also intended to prevent electrically conductive contact occurring between these dissimilar metals. However, as explained below, these coatings are not always effective at preventing conductive contact between the dissimilar metals, with the risk of unsightly and unhygienic “bimetallic corrosion” at locations where conductive contact occurs.
Explained simply, “bimetallic corrosion” is the corrosion that occurs when dissimilar metals come into conductive contact in the presence of an electrolyte. It is also known as galvanic corrosion. In bimetallic corrosion, the corrosion of a reactive metal (the anode) occurs due to positive electric current flowing from the anode to the less reactive (more noble) metal (the cathode) through the electrolyte. This process is similar to the conventional corrosion of a single uncoupled metal, but generally proceeds at a higher rate depending on the difference in the electrochemical reactivity of the anode and cathode metals. In the context of the present invention, “dissimilar metals” therefore mean metals having different electrochemical reactivities such that when they are put into conductive contact in the presence of an electrolyte, bimetallic corrosion can occur.
By way of example, considering the case of an aluminium-based foil lid bonded to a steel/tin-plate ring/container body:
aluminium is more anodic than both conventional carbon steel and tin; consequently, any conductive contact between the aluminium of the foil lid and the dissimilar metal of the ring/container body risks bimetallic corrosion of the aluminium of the lid at those points of contact.
Typically, the first stage in making a foil lid is to cut a blank out of a sheet of pre-coated foil lidding material. Regardless of any coating that may have been pre-applied to the sheet of lidding material, the action of cutting results in a surface of the metal of the foil lid (e.g. aluminium) being exposed at the peripheral cut edge of the lid. During the process of locating the lid against the ring/container body and subsequent bonding, it has been found that any relative movement between the lid and the ring/container body can result in the exposed peripheral metal edge of the foil cutting through any coatings on the ring/container body and thereby establishing direct metal:metal contact. This risk is exacerbated if the ring/container body is used as a forming die to shape the periphery of the lid—as illustrated in FIGS. 9-11 of WO 2006/092364A. In WO 2006/092364A, an inclined region is applied to a planar foil lid blank by drawing the lid blank against a correspondingly inclined surface of a container body, bonding between the container body and foil lid then occurring between the respective inclined regions of the lid and container body. The action of drawing the lid against the container body can easily result in any coating on either the container body or the foil lid being scratched or damaged, thereby exposing the underlying metal of the container body and presenting an additional route by which conductive metal to metal contact will occur. Additionally, subsequent handling and transportation of filled containers is also highly likely to result in scratching and other damage to any coating provided on the ring/container body, thereby exposing the bare metal of the ring/container body. Where these scratches—regardless of how they occurred—are adjacent the exposed peripheral metal edge of the lid, electrolyte in the form of water or other chemicals can easily establish a conductive path between the dissimilar metals of the lid and the ring/container body, and bimetallic corrosion can quickly occur.
In technical fields outside of metal packaging, known ways of mitigating the risk of bimetallic corrosion include:                Galvanizing the least noble metal with a sacrificial metal coating, e.g. as used for protecting car body panels. Whilst technically feasible, it is undesirable for packaging because the galvanising process would increase manufacturing costs.        
The present invention therefore has the objects of:                Providing an improved metal closure having a foil lid sealed to an annular component—the lid and annular component made of dissimilar metals—with a cheap and effective means of reducing the risk of galvanic (bimetallic) corrosion between the dissimilar metals of the lid and the annular component.        Providing a method and apparatus for making such a closure.        